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In addition to her clinical practice in Toronto, Fiona Soong, OD, BSc works in private research, developing models to investigate dry eye syndrome and allergic conjunctivitis using environmental exposure chambers.

Allergic diseases involve a multitude of chemical mediators and inflammatory cells that evolve and change according to patient phenotype and environmental contributors. Ocular manifestations of allergies can be particularly troublesome, with a significant impact on contact lens wear. What triggers this ocular response, and what signs and symptoms can accompany it? Fiona Soong reviews some key points in the following comprehensive article by Mark Abelson, who has written extensively on this subject. The article provides an excellent overview of the pathophysiology of the ocular allergic response, including its activation mechanisms and clinical manifestations. For more about treatments for ocular allergies, visit this edition’s editorial.

Abelson reminds us that mast cells are present in the sub-epithelial layer of the conjunctival stroma and near limbal blood vessels. The binding of antigen-specific IgE antibodies to receptors on these mast cells lead to the release of mediators such as histamine, tryptase and cytokines, within their cell structure. The degranulation of the mast cell also initiates an arachnidonic acid cascade, producing inflammatory mediators like prostaglandins, thromboxanes and leukotrienes.

These chemical mediators stimulate nerve endings, giving rise to an itchy sensation and increasing vascular permeability to cause clinical signs of hyperemia and edema. This reaction can occur hours after as a result of additional inflammatory cells such as eosinophils, neutrophils, basophils and T-lymphocytes. Late phase reactions, although more serious in asthma and rhinitis, result in changes to the ocular surface – keratitis, limbal infiltrates and ulcers.

Allergic conditions

Ocular allergies manifest in a variety of ways and in response to a variety of triggers, each of which is outlined in the article:

Seasonal allergic conjunctivitis (SAC), linked to tree, grass and ragweed pollens, and perennial allergic conjunctivitis (PAC), linked to animal dander and dust mite feces, affect 20% of the population1. The ocular symptoms associated with these types of acute allergies have a significant impact on quality of life2. Primary signs and symptoms are ocular redness and itching, in addition to a watery discharge. Continual exposure to allergens can lead to chemosis and eyelid swelling.

Acute episodes of hyper-reactive conjunctivitis may be triggered by non-specific stimuli such as changes in temperature, wind, fragrance and solar radiation.

Vernal keratoconjunctivitis (VKC) is primarily a lymphocyte-mediated condition that affects patients between 8 and 12 years of age, predominantly males. Most patients outgrow the condition by the age of 30 years.3 Patients complain of extreme itching, stringy mucus discharge; photophobia may be present, secondary to superficial keratopathy and/or ulceration. In the condition’s palpebral form, large papillae develop on the tarsal conjunctiva; in its limbal form, multiple gelatinous nodules along the limbus may be paired with neovascularization.

Atopic keratoconjunctivitis (AKC) results from a continual infiltration of T-cells, eosinophils and cytokines, which may accumulate as Horner-Tranta’s dots along the limbus. Ninety-five percent of patients with this condition tend to have associated eczema; 87% have concomitant asthma. Tylosis, loss of the lateral eyebrow and corneal involvement ranges from micropannus to ulceration to ectasia.

Giant papillary conjunctivitis (GPC) is a delayed hypersensitivity reaction secondary to trauma involving contact between a foreign body and the tarsal conjunctiva, or due to an immune reaction to protein deposits on contact lenses, which rub against the eyelid upon each blink. It is characterized by the presence of large papillae on the tarsal conjunctiva, which can elicit a foreign body sensation and produce excessive mucus discharge, both of which lead to contact lens discomfort.

Drug-induced allergic conjunctivitis is another hypersensitivity reaction secondary to a response to a cosmetic/ophthalmic product, including preservatives, prescribed medications and skin creams. Prolonged ocular residence time or replicate use of ophthalmic drops may cause eye and lid irritation.

As Abelson notes, ocular allergies may not be sight threatening, but they do have a serious impact on the lives of our patients. Identifying and treating these allergic conditions should be taken seriously. Current ophthalmic medications can be immensely helpful in reducing allergic signs and symptoms, with longer duration of action and convenient dosing regimens.